Salmonella and Salmonellosis

Salmonella and Salmonellosis (page 3)

(This chapter has 5 pages)

© Kenneth Todar, PhD

Isolation and Identification of Salmonella

A number of plating media have been devised for the isolation of Salmonella. Some media are differential and nonselective, i.e., they contain lactose with a pH indicator, but do not contain any inhibitor for non salmonellae (e.g., bromocresol purple lactose agar). Other media are differential and slightly selective, i.e., in addition to lactose and a pH indicator, they contain an inhibitor for nonenterics (e.g., MacConkey agar and eosin-methylene blue agar).

The most commonly used media selective for Salmonella are SS agar, bismuth sulfite agar, Hektoen enteric (HE) medium, brilliant green agar and xylose-lisine-deoxycholate (XLD) agar. All these media contain both selective and differential ingredients and they are commercially available.

Figure 3. Salmonella sp. after 24 hours growth on XLD agar. Xylose Lysine (XL) agar is used when trying to culture and isolate Gram-negative enteric bacilli. When XL agar is supplemented with sodium thiosulfate, ferric ammonium citrate, and sodium deoxycholate, it is then termed XLD agar, and is then an even more selective medium than XL alone. The presence of any black colored area indicates the deposition of hydrogen sulfide, (H₂S) under alkaline conditions. (CDC)

Media used for Salmonella identification are those used for identification of all Enterobacteriaceae. Most Salmonella strains are motile with peritrichous flagella, however, nonmotile variants may occur occasionally. Most strains grow on nutrient agar as smooth colonies, 2-4 mm in diameter. Most strains are prototrophs, not requiring any growth factors. However, auxotrophic strains do occur, especially in host-adapted serovars such as Typhi and Paratyphi A.

Figure 4. Colonial growth Salmonella choleraesuis subsp. arizonae bacteria grown on a blood agar culture plate. Also known as Salmonella Arizonae, it is a zoonotic bacterium that can infect humans, birds, reptiles, and other animals. (CDC)

Table 1. Characteristics shared by most Salmonella strains belonging to subspecies I

Motile, Gram-negative bacteria
Lactose negative; acid and gas from glucose, mannitol, maltose, and sorbitol; no Acid from adonitol, sucrose, salicin, lactose
ONPG test negative (lactose negative)
Indole test negative
Methyl red test positive
Voges-Proskauer test negative
Citrate positive (growth on Simmon's citrate agar)
Lysine decarboxylase positive
Urease negative
Ornithine decarboxylase positive
H₂S produced from thiosulfate
Do not grow with KCN
Phenylalanine and tryptophan deaminase negative
Gelatin hydrolysis negative

Figure 5. Colonial growth pattern displayed by Salmonella Typhimurium cultured on a Hektoen enteric (HE) agar. S. Typhimurium colonies grown on HE agar are blue-green in color indcating that the bacterium does not ferment lactose However it does produce hydrogen sulfide, (H₂S), as indicated by black deposits in the centers of the colonies. (CDC). HE agar is the medium designed for the isolation and recovery of fecal bacteria belonging to the family, Enterbacteriaceae. S.Typhimurium causes 25% of the 1.4 million salmonellosis infections a year in the United States. Most persons infected with Salmonella sp. develop diarrhea, fever, and abdominal cramps 12 - 72 hours after infection. The illness usually lasts 4 - 7 days, and most people recover without treatment. However, in some cases, the diarrhea may be so severe that the patient needs to be hospitalized.

Genetics of Salmonella

The genetic map of the Salmonella Typhimurium strain LT2 is not very different from that of Escherichia coli K-12. The F plasmid can be transferred to Typhimurium, and an Hfr strain of Typhimurium may subsequently be selected. Conjugative chromosomal transfer may occur from Typhimurium Hfr to E. coli or from E. coli Hfr to Typhimurium. Chromosomal genes responsible for O, Vi, and H antigens can be transferred from Salmonella to Escherichia.

Also, Salmonella may harbor temperate phages and plasmids. Plasmids in Salmonella may code for antibiotic resistance (resistance plasmids are frequent due to the selective pressure of extensive antibiotic therapy), bacteriocins, metabolic characteristics such as lactose or sucrose fermentation, or antigenic changes of O antigen.

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